Process for the adsorption of organic vapours from gas mixtures containing them

ABSTRACT

The invention is directed to a process for the adsorption of organic vapours from gas mixtures containing them onto activated carbon, where the process comprises passing the gas mixture consecutively through at least a first and a second adsorption system, the first system consisting of activated carbon having a first adsorption rate for the organic vapours and the second system consisting of an adsorbent having a second adsorption rate for the organic vapours, the second adsorption rate being higher than the first adsorption rate, more in particular to such process when used in the operation of vehicle internal combustion engines.

The invention is directed to a process for the adsorption of organicvapours from gas mixtures containing them onto activated carbon.

Emission of organic vapours is harmful to the environment (smog). Theemission from automotive sources is regulated by law worldwide,requiring car manufacturers to take measures to reduce the emission offuel vapours from the fuel tanks during operation or refueling or duringperiods when the car is not driving, for example due to the temperaturechanges (day-night).

These emission reduction systems are based on activated carbon and areregenerated during vehicle operation, when combustion air is passedthrough the system, thereby removing the fuel from the activated carbon.These systems are known as evaporative loss control devices(ELCD-canister). Also it can be useful in stationary systems to applyactivated carbon for removing organic vapours from gas mixtures.Examples are in the area of heating devices and such like.

Current emission limits have resulted in the use of special canisterscontaining specialised types of activated carbon, that are well suitedto reduce the emission sufficiently. In the near future the emissionlimits will be reduced drastically; these levels cannot be met bycurrent systems, especially not during refuelling. In that situation,not all vapours are adsorbed sufficiently fast and bleed through occurs.Increasing the adsorption capacity of the canister by increasing itssize is not acceptable, since the available space for incorporatingthese canisters in a car is limited. Increase of the adsorption capacityper unit carbon volume can only be accomplished at high costs and stillthe adsorption rate will be insufficient.

A solution would be to increase the adsorption kinetics (adsorptionrate), however, the common way to do that results in other problems.Usually this is done by decreasing the particle size of the adsorbent,but this results in an increase in pressure drop, which makes itdifficult to properly refill the gasoline tank, i.e. without untimelystopping of the gasoline pump of the service station. An otheralternative is to change the external surface area to volume ratio (Sv,with dimension of e.g. m²/m³) of the adsorbent particles by selecting adifferent external shape. The consequence of this is a smaller amount ofadsorbent in the same volume (i.e. a decrease in density), which is notacceptable, as the total adsorption capacity decreases.

Accordingly it is an object of the present invention to provide aprocess for the adsorption of organic vapours, in particular organicvapours (hydrocarbons) that are held responsible for smog formation,from gas mixtures containing them onto activated carbon, which processdoes not have the drawbacks described above. In the present descriptionand claims, organic compounds that are held responsible for smogformation are gaseous compounds originating from fuels and areparticularly defined as hydrocarbons having more than three carbonatoms. More in particular these compounds are present in the vapours offuels such as gasoline, kerosene or fuel oil.

The process of the present invention should be suitable for applicationin ELCD-canisters. A further object is to provide for an improvement inthis respect, i.e. to provide an ELCD-canister which enables a decreaseof the emission of smog-forming hydrocarbons, without a significantincrease in the pressure drop across the canister and without asignificant decrease in the canister's adsorption capacity. According tothe invention the process comprises passing the gas mixtureconsecutively through at least a first and a second adsorption system,the first system consisting of activated carbon having a firstadsorption rate for the organic vapours and the second system consistingof an adsorbent having a second adsorption rate for the organic vapours,the second adsorption rate being higher than the first adsorption rate.

In a preferred embodiment said process is applied to automotive systems,wherein the adsorption system is operated in relation to an internalcombustion engine.

The present invention is based on the surprising insight, that by theuse of two consecutive adsorption systems with comparable adsorptioncapacities per unit volume, a first one having a limited adsorption rateand preferably a low pressure drop, and a second one having a higheradsorption rate, the emission can be decreased, without having to resortto other measures, such as increasing the size of the (standard)adsorption canisters, to eliminate the bleed through.

By the use of this system of at least two combined adsorbents havingdifferent adsorption kinetics, a better use of the available adsorptioncapacity is made, without an undue increase of canister size or pressuredrop over the system.

According to the invention it was thus found that a canister containinga limited amount of adsorbent with fast adsorption kinetics at theoutlet side of a regular canister already provides an improved reductionin emission. Apparently, the mass transfer zone obtained in thedownstream adsorbent, preferably an activated carbon or zeolite bed, wasreduced in size in this downstream section, thus increasing theefficiency.

In a preferred embodiment the present invention comprises the use of anactivated carbon filter containing

-   -   1. activated carbon having a relatively low pressure drop and a        high adsorption capacity per unit volume but limited adsorption        kinetics, combined with    -   2. a second activated carbon with higher adsorption kinetics,        for example by using smaller particles or other particle shapes.

The combining of both materials in a regular ELCD-canister ensures thatthe best aspects of both options are combined. The high adsorptioncapacity and low pressure drop of the regular carbon is combined withthe fast kinetics of the downstream carbon. Since the carbon with thefast adsorption kinetics is placed at the outlet of the canister thebleeding of vapour is minimized. Accordingly, the amount of downstreamcarbon with higher adsorption kinetics is limited compared to the amountof regular carbon, thereby minimizing the pressure drop over thecomplete filter.

In the operation of the present invention there are variouspossibilities for the application thereof. In a first, preferred,embodiment, both adsorbents are placed in the same canister, either indirect contact with each other or separated by a grid, giving a singleadsorbent bed, containing two different adsorbents with differentproperties as discussed above. It is also possible to place eachadsorbent in a separate canister, the canisters being mutually connectedall the time. In a third embodiment it is possible to place bothadsorbents in separate canisters or separate sections of a singlecanister, which are connected to each other using valves which allow thegasoline vapours to enter the second, downstream canister or canistersection only when required.

As indicated above, the present invention is based on the use of twodifferent adsorbents, the first one being a regular activated carbon. Assecond adsorbent it is possible to use activated carbon, but one canalso use other adsorbing materials which can be regenerated by passingthe air that is used in the internal combustion engine through theadsorbent. Preferred materials are activated carbon and zeolites. Incase of use of activated carbon as second adsorbent, it is to be notedthat this must have different adsorption kinetics from the first carbon.This can for example be realized by using the same activated carbon asin the first adsorbent, but having different surface to volume ratio,thereby changing the diffusion characteristics of the activated carbon.One possibility is to make the particle size of the carbon smaller,whereas another possibility is to change the outer shape, for exampleusing specifically shaped extrudates or tablets. Another possibility toimprove the adsorption kinetics is to select a carbon type with a moreoptimal pore size distribution, thus improving the diffusioncharacteristics. A third possibility is the use of fibrous activatedcarbon, e.g. activated carbon felts or cloths, or the use of activatedcarbon containing fibrous materials, which exhibit high adsorptionkinetics combined with a low pressure drop and a relatively lowadsorption capacity.

For practical purposes, it is preferred to use extruded activated carbon(EAC) as the particles having a low Sv and granular activated carbon(GAC) as the particles with the high Sv. Preferably, EAC having a Sv ofabout 1250-7000 m²/m³, more preferably 2000-3250 m²/m³ is used. The Svof the GAC particles is preferably from 2000-30 000 m²/m³, morepreferably from 3000-12 000 m²/m³.

According to the invention, the amount of adsorbents is selected such,that the adsorption capacity for gasoline vapour of the first adsorptionsystem is larger than the adsorption capacity of gasoline vapour of thesecond adsorption system. Generally, this means that the amount, byweight, of the second adsorbent will be lower than the amount, byweight, of the first adsorbent.

As indicated above, the present invention is applicable for theadsorption of organic vapours from gas mixtures, more in particular forthe removal of gasoline or fuel oil vapours from gas mixturesoriginating in relation to the operation of vehicle internal combustionengines. Although the main application of the invention certainly liesin the so-called evaporative loss control devices (ELCD-canisters) whichare designed to prevent emission of gasoline and fuel oil vapours causedby “breathing” of gasoline and fuel oil tanks, it is also possible touse the invention in relation to stationary tanks containing organicmaterials having a measurable vapour pressure at ambient temperature,such as tanks for gasoline, fuel oil, kerosene and naphtha.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows breakthrough curves of activated carbon beds containing EACand GAC particles in ratios 95/5 and 90/10 (according to the invention)and 100/0 and 0/100 (reference), respectively.

FIG. 2 shows hydrocarbons emission vs. outlet hydrocarbonsconcentration, as measured with activated carbon beds containing EAC andGAC particles in ratios 95/5 and 90/10 (according to the invention) and100/0 and 0/100 (reference), respectively.

The present invention is now elucidated on the basis of three examples.

COMPARATIVE EXAMPLE

A standard evaporative loss control device canister, containingactivated carbon, 2 mm/diameter extrudate was used and the emission inthe “three day diurnal test” (US-EPA test procedure) was determined. Theemission value obtained was set as 100%.

EXAMPLE 1

Using the same method as in the comparative example, the emission wasdetermined of the same ELCD canister, containing 80% (by weight)activated carbon, 2 mm/diameter extrudates and 20% by weight granularactivated carbon (sieve fraction 0.5-2.0 mm), downstream from theextrudate was used. In the same test procedure as used in thecomparative example the emission was determined. The emission valuesobtained were 30% of the values obtained in the comparative example.

EXAMPLE 2

The performance of activated carbon for evaporative loss control wastested in a dedicated, automated test apparatus, in which a carbon bedis loaded with a model gasoline vapour and regenerated with air duringvarious cycles. The applied test is a simulation of the conditions thatare imposed on the carbon in an ELCD canister in practise.

The diameter of the test tube containing the carbon bed was 3.6 cm. Thecarbon bed height was always 15 cm, resulting in a total carbon volumeof 153 cm³. The test tube was placed in a thermostatically controlledchamber at a temperature of 30° C. during the complete test.

The model gasoline vapour consisted of 50 vol % air, 33 vol % butane and17 vol % of a mixture of various hydrocarbons (HC-mix). The compositionof this HC-mix is presented in Table 1.

The contact time in the carbon bed was 75 seconds during each loadingstep. The total hydrocarbons concentration in the off-gas was measuredevery 10 seconds during the complete loading step using a flameionisation detector (FID). The loading step was immediately stopped andthe regeneration step was started once the measured FID-signal becamehigher than the FID-signal corresponding to 440 ppmv methane. The timerequired to reach this signal is taken as the loading time. Thehydrocarbons concentration as measured by the FID is expressed as ppmmethane equivalent, indicating the methane concentration giving the sameFID-signal.

Regeneration was always performed in counter-flow (downstream) during 16minutes, using 300 bed volumes of dry air.

Part of an extruded activated carbon sample (2 mm diameter) was crushedand sieved to obtain a sieve fraction containing particles between 1-2mm in diameter. The surface to volume ratio (Sv) of these particles was4000 m²/m³. This sieve fraction is indicated as granular activatedcarbon (GAC) hereafter. A multi-layer carbon bed was created by fillingthe test tube with extruded activated carbon (EAC, having an Sv of 2400m²/m³) first, after which an additional layer of GAC was added in such away that the total activated carbon bed volume remained unchanged (153cm³). The ratio EAC/GAC was varied as follows: 100/0, 95/5, 90/10,0/100.

After the performance test the pressure drop in air over the carbon bedsdescribed above was measured at various linear flow rates.

By default, each carbon bed was tested for 75 loading and regenerationcycles under identical test conditions.

The gasoline working capacity (GWC) is defined as the effectiveadsorption capacity for hydrocarbons under the mentioned testconditions. The GWC was calculated for each cycle as follows.${GWC} = {\frac{\left( {{Loading}\quad{Time}} \right) \cdot \left( {{HC}\quad{Supply}\quad{Rate}} \right)}{1000 \cdot \left( {{Carbon}\quad{Volume}} \right)}\left\lbrack {{g/100}\quad{ml}} \right\rbrack}$

Due to ageing the GWC usually slightly decreases in time. The resultsobtained in cycle 75 are assumed to represent the performance afteraging of the carbon and are the most close to a steady state situationin practise.

The outlet hydrocarbons concentration was registered as a function oftime in order to establish the emitted amount of hydrocarbons during theloading step of the carbon. These data resulted in the so-calledbreakthrough curves for each loading step. By integration of thebreakthrough curves the total amount of emitted hydrocarbons wasestablished.

In FIG. 1 the breakthrough curves in cycle 75 are presented for thevarious carbon beds described above, containing EAC/GAC in the ratios100/0, 95/5, 90/10, and 0/100. FIG. 1 clearly shows that thehydrocarbons concentration in the off-gas with EAC only is at a muchhigher level than that observed with GAC only, thus resulting in highertotal emission values. It follows that the adsorption rate of the GACparticles is higher than the adsorption rate of the EAC particles Alsothe multi-layer carbon beds, containing a certain fraction GAC at theoutlet side of the carbon bed, show a low hydrocarbons concentration inthe off-gas. Furthermore, breakthrough of the 95/5 and 90/10 bedsoccurred later than with the 100/0 and 0/100 beds.

Table 2 contains the loading times in cycle 75 established using thementioned carbon beds, relative to the loading time of the carbon bedcontaining EAC only. These data indicate that the total loading times ofthe multi-layer carbon beds are at least as long as that of the carbonbed containing EAC. This shows that the performance of the carbon bed(GWC) is not affected in a negative way by applying a multi-layer bed.The shorter loading time of the GAC bed primarily resulted from thelower packed density of this carbon compared to EAC, thus reducing theamount of activated carbon in the test tube.

FIG. 2 represents the test results of cycle 75 after integrating thebreakthrough curves. It shows the cumulative hydrocarbons emission as afunction of the outlet hydrocarbons concentration. From FIG. 2 itfollows that the hydrocarbons emission for a multi-layer carbon bed isat the same level as that of a bed filled with GAC only, and at a muchlower level than that of a bed filled with EAC only.

Table 3 contains the pressure drop data in air over the carbon beds atvarious linear gas flow rates. These data indicate that the pressuredrop of the multi-layer carbon bed is only slightly higher than that ofa carbon bed containing only EAC, and considerably lower than that of acarbon bed containing GAC only.

TABLE 1 Composition of the HC-mix used to compose a model gasolinevapour. Content in HC-mix Hydrocarbon Compound [mole fraction] n-pentane0.579 n-hexane 0.077 1-hexene 0.050 benzene 0.013 toluene 0.053 2,3di-methylpentane 0.017 iso-octane 0.043 ethylbenzene 0.017 o-xylene0.014 nonane 0.006 MTBE 0.131

TABLE 2 Loading times during cycle 75 of carbon beds containingdifferent amounts of EAC and GAC. Carbon Bed Composition LoadingTime/Loading Time using EAC only [ratio EAC/GAC] [%] 100/0  100.0 95/5 100.7 90/10 100.4  0/100 96.1

TABLE 3 Pressure drop in air over carbons beds containing differentamounts of EAC and GAC. Pressure Drop in Air [kPa/m] Linear Air FlowEAC/GAC- EAC/GAC-ratio EAC/GAC-ratio Rate [cm/sec] ratio 100/0 90/100/100 7.7 0.4 0.5 1.2 15 1.3 1.7 3.1 23 2.4 3.0 5.4 31 3.7 4.6 8.5 385.7 6.9 12 46 7.7 9.2 17 54 11 12 21 61 14 17 27 69 17 20 34

1. Process for the adsorption of smog-forming hydrocarbon vapours fromfuel gas mixtures containing them onto activated carbon, said processcomprising passing the gas mixture consecutively through at least afirst and a second adsorption system, the first system consisting ofextrudated activated carbon (EAC) having a first adsorption rate fororganic vapours and the second system consisting of activated carbonhaving a second adsorption rate for organic vapours, the secondadsorption rate being higher than the first adsorption rate.
 2. Processaccording to claim 1, wherein said EAC has an Sv of 1250-7000 m²/m³. 3.Process according to claim 2, wherein the particle size of the activatedcarbon in said second adsorption system is smaller than the particlesize of the activated carbon in said first adsorption system.
 4. Processaccording to claim 2, wherein said activated carbon from said secondsystem has an Sv of 2000-30,000 m²/m³.
 5. Process according to claim 4,wherein: the activated carbon in both adsorption systems is based on thesame material, with different ratio of external surface to volume; theparticle size of the activated carbon in said second adsorption systemis smaller than the particle size of the activated carbon in said firstadsorption system; the pressure drop per unit bed depth of the adsorbentin said first adsorption system is lower than the pressure drop per unitbed depth of the adsorbent in said second adsorption system; and saidfuel gas mixtures are selected from gases originating from gasoline,kerosene or fuel oil.
 6. Process according to claim 1, wherein saidactivated carbon for the second system has an Sv of 2000-30,000 m²/m³.7. Process according to claim 6, wherein the activated carbon in bothadsorption systems is based on the same material, with different ratioof external surface to volume.
 8. Process according to claim 1, whereinthe pressure drop per unit bed depth of the adsorbent in said firstadsorption system is lower than the pressure drop per unit bed depth ofthe adsorbent in said second adsorption system.
 9. Process according toclaim 1, wherein said fuel gas mixtures are selected from gasesoriginating from gasoline, kerosine and fuel oil.
 10. Process accordingto claim 1, wherein said activated carbon of the second system isgranular.
 11. Process for the removal of gasoline or fuel oil vapoursfrom gas mixtures originating in relation to the operation of vehicleinternal combustion engines, said process comprising passing the gasmixture consecutively through at least a first and a second adsorptionsystem, the first system consisting of extrudated activated carbon (EAC)having a first adsorption rate for organic vapours and the second systemconsisting of activated carbon having a second adsorption rate fororganic vapours, said second adsorption rate being higher than saidfirst adsorption rate.
 12. Process according to claim 11, wherein saidEAC has an Sv of 1250-7000 m²/m³.
 13. Process according to claim 12,wherein the particle size of the activated carbon in said secondadsorption system is smaller than the particle size of the activatedcarbon in said first adsorption system.
 14. Process according to claim12, wherein said activated carbon from said second system has an Sv of2000-30,000 m²/m³.
 15. Process according to claim 14, wherein: theactivated carbon in both adsorption systems is based on the samematerial, with different ratio of external surface to volume; theparticle size of the activated carbon in said second adsorption systemis smaller than the particle size of the activated carbon in said firstadsorption system; the pressure drop over the adsorbent in said firstadsorption system is lower than the pressure drop over the adsorbent insaid second adsorption system; the loaded adsorption systems areregenerated by passing combustion air through the adsorption systems.16. Process according to claim 11, wherein said activated carbon fromsaid second system has an Sv of 2000-30,000 m²/m³.
 17. Process accordingto claim 16, wherein the activated carbon in both adsorption systems isbased on the same material, with different ratio of external surface tovolume.
 18. Process according to claim 11, wherein the pressure dropover the adsorbent in said first adsorption system is lower than thepressure drop over the adsorbent in said second adsorption system. 19.Process according to claim 11, wherein the loaded adsorption systems areregenerated by passing combustion air through the adsorption systems.20. Process according to claim 11, wherein said activated carbon of thesecond system is granular.
 21. Process for the removal of gasoline,kerosene or fuel oil vapours from gas mixtures originating in relationto the operation of vehicle internal combustion engines, said processcomprising passing the gas mixture consecutively through at least afirst and second adsorption system, the first system adsorption rate fororganic vapours and the second system consisting of activated carbonhaving a second adsorption rate for organic vapours, said secondadsorption rate being higher than said first adsorption rate, whereinthe loaded adsorption systems are regenerated by passing combustion airthrough the adsorption systems.
 22. Process according to claim 21,wherein said EAC has an Sv of 1250-7000 m²/m³.
 23. Process accordingclaim 22, wherein the particle size of the activated carbon in saidsecond adsorption system is smaller than the particle size of theactivated carbon in said first adsorption system.
 24. Process accordingto claim 22, wherein said activated carbon from said second system hasan Sv of 2000-30,000 m²/m³.
 25. Process according to claim 21, whereinsaid activated carbon from said second system has an Sv of 2000-30,000m²/m³.
 26. Process according to claim 25, wherein the activated carbonin both adsorption systems is based on the same material, with differentratios of external surface to volume.
 27. Process according to claim 21,wherein the pressure drop over the adsorbent in said first adsorptionsystem is lower than the pressure drop over the adsorbent is said secondadsorption system.
 28. Process for the removal of gasoline, kerosene orfuel oil vapours from gas mixtures originating in relation to theoperation of vehicle internal combustion engines, said processcomprising passing the gas mixture consecutively through at least afirst and a second adsorption system, the first system consisting ofextrudated activated carbon (EAC) having a first adsorption rate fororganic vapours and the second system consisting of activated carbonhaving a second adsorption rate for organic vapours, said secondadsorption rate being higher than said first adsorption rate, whereinthe loaded adsorption systems are regenerated by passing combustion airthrough the adsorption systems and the gas mixtures are selected fromgases originating from gasoline, kerosene or fuel oil.